It is well understood in the vehicle fuel storage and delivery art that it is necessary to vent a vehicle fuel tank to relieve vapor pressure build-up which may occur when the filler pipe cap is in place and to permit the escape of air during normal filling operations when the filler cap is removed. Environmental control laws and regulations require that vehicle fuel tank systems include safeguards to prevent the excessive escape of fuel vapors under normal operating conditions and to prevent the escape of raw fuel from the tank in the event of a roll-over accident or the assumption of an abnormal vehicle attitude.
In this connection it is well known to vent the fuel system to a vapor trap such as a carbon canister. It is important to ensure that fuel vapors and/or raw fuel from the tank does not flood the canister during normal vehicle operation or during filling. This can be achieved with a device known as a "head valve"; such a device is disclosed in U.S. Pat. No. 4,753,262 issued Jun. 28, 1988 and assigned to the assignee of this patent. Another rollover valve is shown in U.S. Pat. No. 4,095,609 issued Jun. 20, 1978 and assigned to Chrysler Corporation.
While excessive pressure in the tank is the more commonly recognized problem, it is also true that a vacuum or negative pressure can be created in the interior of a fuel tank upon rapid cooling of the heated vapor volume contained herein. If this negative pressure condition is not relieved, even a slight pressure drop acting over the interior surface of the fuel tank can cause it to buckle or collapse. This can change fuel capacity, decalibrate the fuel gage and create a fuel spill hazard.